Mac
Temporarily silences your Mac’s system alerts so you can work without getting distracted by popup windows or chiming notifications. This one's great for quieting your machine when presenting a deck at a meeting or for silencing the Mac for 30-minute work sprints.

Android
Does the same thing as Hush, but for your Android phone. Set the timer for 15 minutes or 2 hours or indefinitely, and when the timer runs out, your phone goes back to its default sound settings.

Chrome Extension
Allows you to focus on only one Google Chrome tab, while temporarily hibernating every other open tab. One click restores everything you had open once you decide your work is done. It's a perfect tool if you work within your browser a lot and need to cut out the temptation to check Facebook or Gmail.

Mac
A downloadable tool that can turn a web app such as Wrike (or Gmail, or Evernote, etc.) into a standalone OSX desktop app that you can place in the dock. It's almost like an internet shortcut in that one click launches the web app. This cuts down on the possibility of getting sucked into your Twitter feed (though you could always make a Fluid app just for Twitter). Tip: hide the status bar and the toolbar, so you forget you're actually using a browser -- less chances of typing in a new URL.

Mac
A beautifully-designed fullscreen word processor with built-in audio tracks and keyboard typing sound effects. (If you don't like the new-age music or sound effects, turn them off and use your own music.) While the older version was freeware, the current version is now pay-what-you-want (they do have suggested amounts). Although, you can still find the old free version via your favorite search engine.

Mac/Win/Lin
An open source (free) writing tool available for Mac, Windows and Linux! It can do everything Dark Room does, and pretty much everything OmmWriter does (no audio however and it isn't very appealing graphics-wise). But hey, stop complaining: after all it’s open source, and multi-platform.

Mac
A simple app that allows you to play three different versions of coffee shop ambience (basically the background sounds of a coffee shop). Some people swear it helps you focus and block out distractions.

Browser
A website for listening to (or assembling) playlists of audio tracks of every genre of music imaginable. Do a search for playlists with tags such as "focus+work," "study," or "productivity" to find curated music that may get you in the flow. Or check out our two favorite playlists: Relax to Infinity and Welcome to the Grind.a

Browser/Android/iOS
A work management tool that can hold everything from your grocery lists to more gargantuan projects involving people from different departments, regions, and even countries. Organizing your work means your tasks can be prioritized, delegated, and assigned. It's also the best place to log all your ideas, and schedule all your project plans. making it the central hub for everything you have to do.